Fix clipboard request on macOS for yank-media

Fix clipboard request on macOS for yank-media

'yank-media' makes a request with 'CLIPBOARD and 'TARGETS, which on
other platforms returns the type(s) of the current selection, but on
macOS returned the selection itself.  Make it return the types instead.

This fixes (Bug#71377).

* src/nsselect.m (ns_get_local_selection): Return nil when called with
QCLIPBOARD and QTARGETS, so that 'ns_get_foreign_selection' is called to
return the type(s) of the selection.